This invention relates generally to interlocking intramedullary nails and, more particularly, to a tubular interlocking intramedullary nail having variations in wall thickness along its length.
The use of intramedullary nailing in the treatment of fractures of the femur, tibia, and other "long" bones is well known. This practice can allow a fracture patient to resume limited use of the affected body part within days of the injury and subsequent surgery. The corresponding reduction in the amount of time during which the patient must be at least partially immobilized can drastically reduce the overall recovery period.
One of the earliest forms of intramedullary nails to achieve relative widespread acceptance and usage is often referred to as the Kuntscher nail (or K-nail) for its developer Professor Gerhart Kuntscher of Hamburg, Germany. The Kuntscher nail is a slotted steel tube having a relatively thin side wall thickness which allows the nail to bend or flex slightly as it is driven into the somewhat curved medullary canal of a bone. The Kuntscher nail is also transversely elastic and is approximately the same diameter as the medullary canal into which it is to be driven, causing the nail to be compressed against the sides of the canal and firmly locked in place by compressive forces acting along the length of the nail.
Despite the widespread success and acceptance of the Kuntscher nail, it has been recognized that standard Kuntscher nailing is contraindicated in the treatment of certain complex types of fractures. These fractures are often treated by dynamic or static locking of the nail on one or both ends by screws that extend transversely through the nail and into the major fragments of the fractured bone. This interlocking technique requires the provision of transverse holes or openings through the proximal and/or distal portions of the interlocked nail. Intramedullary nails rarely fail by stress fatigue when no interlocking screws are used. However, the transverse openings and screws of the interlocking nail produce the potential for high concentrations of stress at the proximal and distal ends of the nail, and numerous instances of fatigue failure of interlocking nails have been recorded and analyzed (see, for example "Fatigue Fracture of the Interlocking Nail in the Treatment of Fractures of the Distal Part of the Femoral Shaft" by R. W. Bucholz, M.D., S. E. Ross, M.S. and K. L. Lawrence, Ph.D, P.E., The Journal of Bone and Joint Surgery, Vol. 69-A, No. 9, December, 1987, pp. 1391-1399).
An object of the present invention is to provide an interlocking intramedullary nail which is more resistant to fatigue fractures in the proximal and distal portions of the nail.
Another object of the present invention is to provide an interlocking intramedullary nail which is not only resistant to fatigue fractures, but which retains the desirable flexibility and resilience of the standard Kuntscher nail.
Yet another object of the present invention is to provide an especially advantageous method of manufacturing an interlocking intramedullary nail having the just-described desirable characteristics.
These and other objects of the invention are attained in an intramedullary nail having a proximal portion, a distal portion, and an intermediate portion between the proximal and distal portions, and having at least one opening extending transversely through at least one of either the proximal or distal portions. The nail comprises a unitary piece of elongate material of tubular cross-section having variations in wall thickness along its length. The wall thicknesses of the proximal and distal portions are substantially greater than the wall thickness of the intermediate portion. In an especially preferred embodiment, the wall thicknesses of the proximal and distal portions are approximately twice the thickness of those of the intermediate portion. In one preferred embodiment of the invention, the outside diameters of the distal and intermediate portions are substantially equal, while the outside diameter of the proximal portion is substantially larger. In an alternative embodiment, the outside diameters of the proximal, intermediate and distal portions are substantially equal. In especially preferred embodiments, the nail is provided with a longitudinal slot which extends along substantially the entire length of the nail, and the proximal end of the nail is provided with internal threads to provide for attachment of insertion and extraction devices. The substantially thicker cross-section of the proximal portion of the nail allows such features to be incorporated into the nail design, while maintaining the required strength and mechanical integrity to resist fatigue failure during use and to avoid problems during insertion and extraction.
Another aspect of the present invention relates to the provision of internal threads in the proximal end of the nail for attachment of insertion and extraction devices. It is not uncommon for intramedullary nails of the present type to remain within the body for a number of months, prior to being removed after the fractured bone has healed. During such extended periods, tissue growth around and into the threaded end portion of the nail can occur. Prior to removing the nail, it is necessary for the surgeon to remove such tissue growth to expose the internal threads to allow for attachment of an extraction tool. This is often done by cutting or drilling through the tissue that has grown into the nail, at the risk of damaging surrounding tissue, the nail end, or the internal threads. Accordingly, the nail of the present invention includes a threaded end cap whose function is to seal the end of the nail and the internal threads to prevent tissue ingrowth into these areas.
An especially advantageous method of making an intramedullary nail having variations in wall thickness along its length includes a two-step process to produce the basic overall shape of the nail. The process preferably begins with a single unitary length of tubing having a substantially constant inside diameter, outside diameter and wall thickness. For purposes of this discussion, the length of tubing can be said to have a proximal portion, a distal portion, and an intermediate portion between the proximal and distal portions. The first step in the preferred process is machining the intermediate portion of the length of tubing to reduce the outside diameter and wall thickness thereof, while maintaining the inside diameter substantially constant. The second step in the process is swaging either the proximal portion or the distal portion, or both, to reduce the inside and outside diameter(s) thereof, such that the wall thickness of the swaged portion of the tubing is substantially greater than the wall thickness of the intermediate portion of the tubing. As noted elsewhere in this application, one embodiment of the present invention is swaged such that the outside diameters of the proximal, intermediate, and distal portions are all substantially equal. An alternative embodiment results in a proximal portion which has a slightly larger outside diameter than do the intermediate and distal portions.
Additional machining steps may be performed on the distal and proximal portions of the tubing, either prior to or subsequent to the swaging step. Additional steps may also be performed to provide other features, such as the longitudinally extending slot, transverse openings, and internal threads in the proximal end of the tubing.
Although the preferred sequence of the two-step process is as illustrated in the figures described below, variations from this sequence which would produce substantially the same result are possible. Additionally, although the preferred method of manufacture begins with a single length of tubing, a single length of solid stock (or other material type) may also be used. If a single piece of solid stock is used, the machining step will include drilling at least a portion of the length of stock to produce a tubular cross-section in that portion, followed by the machining and swaging steps described above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.